heatline combi pilot light fault?

My heatline combi boiler has developed a fault, I have recently fitted a new bathroom which has involved draining down the system a couple of times. The boiler itself should be in good nick as an engineer looked at it 9 months ago and repaced the diverter valve and main heating element, replaced all associated washers etc and gave it a fresh bill of health. However recently when calling for hot water, the pilot light will fire up fine and hot water will start to flow, 20-30 secs later the light will go out, 10 secs later it will fire up again and this cycle will continue for as long as hot water is called for, resulting in intermitent hot water. Is it simple matter of the system needing another drain down (properly), or does anyone know of anything more serious that could be wrong?

Kind Regards
Ed